It is often necessary to control the radius of curvature of a mirror in order to control precisely the location of the focal point occurs. This is important, for example, when using laser beams for machining or to compensate for changes in other optical components due to heating or other physical changes.
Among the various methods commonly used to achieve such control of the radius of curvature of a mirror are those that utilize pressure on the rear surface of the mirror while the periphery of the mirror is held in a constant position, thus resulting in the desired corrective distortion of the mirror. Such methods include those disclosed in: U.S. Pat. No. 6,021,153 to Okada, issued Feb. 1, 2000 that describes a variable-curvature reflecting mirror in which the radius of curvature is adjusted with a piezoelectric actuator that applies pressure against the back of the mirror; U.S. Pat. No. 6,253,619 to Danyluk et al., issued Jul. 3, 2001 that describes an adjustable acoustic mirror in which the radius of curvature is adjusted with a screw, rod, or voltage modulator; and U.S. Pat. No. 6,464,364 to Graves et al., issued Oct. 15, 2002 that describes a deformable curvature mirror in which the deformation is controlled by applying electrical voltages to electrode segments on the back of the mirror.
While each of these and similar prior art devices attempt to provide control of the radius of curvature of a mirror, they are substantially complex mechanically and consequently expensive to implement and often do not provide the desired perfectly spherical distortion of the mirror.
There is therefore, a need for a relatively simple radius of curvature control system that provides nearly spherical distortion of the mirror in response to external influences, especially heating such as that caused by laser beams.